The multi-channel cochlear implant was first implanted in 1978. Early signal processing designs extracted the second formant (F2) and pitch (F0) to control electrode stimulation. The frequency of F2 controlled the location of electrode stimulation, and F0 controlled the rate of stimulation. This was later improved by also extracting the first formant (F1) and adding a second stimulated electrode for each pitch period. The MULTI-PEAK (MPEAK) stimulation strategy added stimulation of a number of fixed electrodes to better represent high-frequency information. The next stages of development were the SMSP and SPEAK strategies. These were a departure from the others at they used a fixed stimulation rate and stimulated electrodes that corresponded to maxima in the sound spectra. Another fixed-rate strategy, CIS, was developed overseas. This strategy stimulated all of a small number of electrodes to represent the sound spectra. All of the above processing strategies involve fixed-rate sound processing.
The named inventors have determined that some speech features are better perceived using low-rates of simulation, while some are better perceived using high rates of stimulation. Higher rates of stimulation present more information about phonetic manner of articulation, but spectral information tends to be smeared at such higher rates.